The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
When Do Mounjaro Side Effects Start?
Side effects from Mounjaro (tirzepatide) typically begin within the first few days to weeks of starting treatment, though timing and severity vary significantly between individuals. Most people experience nausea or digestive changes earliest, often within 24–72 hours of the first 2.5 mg injection. Understanding what to expect helps you manage side effects effectively. Our GLP-1 weight loss calculator can help you explore what results and timelines might look like with treatment.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Mounjaro vs Wegovy: Side Effect Timing
Mounjaro side effects typically emerge faster than Wegovy for most people. This is partly due to Mounjaro's dual GIP and GLP-1 mechanism, which affects the body in two ways simultaneously. Common side effects—nausea, constipation, and loss of appetite—often appear within 3–7 days of the first injection at 2.5 mg starting dose.
With Wegovy (semaglutide), a GLP-1-only agonist, side effects may develop slightly more gradually. Since Wegovy's starting dose (0.25 mg) is lower than Mounjaro's, initial side effects can take 1–2 weeks to manifest in some cases, though nausea still typically occurs early.
Both medications follow a dose titration schedule—meaning doses increase every 4 weeks. Side effects often worsen temporarily when doses increase, then improve as your body adjusts over days to weeks. Individual tolerance varies greatly based on genetics, stomach sensitivity, and overall health.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
What Timeline Should You Expect?
In the first week, nausea is the most commonly reported side effect, affecting more than 1 in 10 people. You may also notice reduced appetite or mild stomach discomfort. These early effects are usually mild to moderate.
By weeks 2–4, gastrointestinal effects (constipation or diarrhoea) often emerge as the main concern. Many people find these settle as their body adjusts to the medication and as they establish consistent meal patterns.
Important: side effect severity depends on your individual biology, not the medication's strength. A prescriber will monitor your response at each dose increase. If side effects are intolerable, your dose may be paused or adjusted. Use our BMI calculator and weight converter to track baseline measurements—this helps your healthcare team assess overall progress against side effect burden.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Do Mounjaro side effects start immediately?
Most people notice nausea or appetite loss within 24–72 hours of the first Mounjaro injection. However, some people experience no side effects in the first week. Severity depends on individual sensitivity. Your prescriber can advise based on your personal health history.
Is Wegovy gentler than Mounjaro for side effects?
Wegovy may feel slightly gentler initially due to its lower starting dose (0.25 mg vs Mounjaro's 2.5 mg), but both cause similar side effect types. Nausea, constipation, and appetite loss occur with both. Timing and severity are highly individual, not medication-dependent.
When do Mounjaro side effects go away?
Most early side effects (nausea, appetite loss) improve within 1–2 weeks as your body adjusts. Gastrointestinal effects may persist longer. Some people experience mild nausea throughout treatment. If side effects don't improve after 2 weeks, speak to your prescriber about dose adjustment or management strategies.
Can I manage Mounjaro side effects at home?
Yes. Eat smaller, frequent meals; stay hydrated; avoid fatty foods; and take anti-nausea medication if approved by your prescriber. Ginger, peppermint tea, and slow eating can help. Keep a food and symptom diary—this helps identify triggers and informs your prescriber's decisions.
Should I switch from Mounjaro to Wegovy if side effects are bad?
Any switch between medications must be managed by your prescriber. Side effects often improve naturally within weeks as your body adjusts. Your prescriber may pause doses, adjust timing, or modify your diet strategy before recommending a switch. Individual response to each medication differs significantly.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.